If your Asus ZenBook S (UX391) no longer holds a charge, you are not alone. Battery degradation is common over time, resulting in reduced unplugged use and frequent charging. (ASUS Battery Information Center, 2024)
If your laptop shuts down unexpectedly or the battery drains quickly, it may be time for a replacement. This guide explains how to identify the correct Asus UX391 battery, install it, and maintain its longevity.
By the end of this article, you will be equipped to restore your Asus ZenBook’s portability and performance.
Identifying the Right Asus UX391 Battery
Before you can replace your battery, you need to make sure you're buying the correct part. The Asus ZenBook S UX391 series uses a specific battery model, and installing the wrong one could damage your laptop or simply not work.
Finding the Battery Model Number
The Asus ZenBook S UX391 typically uses the C21N1725 battery. (C22N1720 Battery for ASUS ZenBook S UX391 UX391UA - 7.7V 50Wh Li-ion Ultrabook Battery, n.d.) To confirm, check the model number printed on your current battery by opening the laptop’s back panel.
Here’s how you can do it safely:
- Power down completely: Shut down your laptop and unplug it from the charger.
- Follow these steps to check safely:ce and have a small Phillips-head screwdriver and a plastic prying tool ready.
- Remove the back cover: Flip your laptop over. You'll see several small screws around the perimeter of the bottom case. Carefully unscrew them.
- Pry open the case: Use a plastic prying tool or a credit card to gently separate the bottom cover from the main chassis. Start at a corner and work your way around to avoid breaking the plastic clips.
- Locate the battery: Once the cover is off, the battery is the large, flat component taking up a significant portion of the internal space. The model number, C21N1725, should be clearly printed on its label.
Key Specifications to Check
When purchasing a new C21N1725 battery, verify these key specifications to ensure compatibility and performance:
- Voltage: The standard voltage is 7.7V. (C22N1720 Battery for ASUS ZenBook S UX391 UX391UA - 7.7V 50Wh Li-ion Ultrabook Battery, n.d.)
- Capacity: Capacity is usually rated in watt-hours (Wh) or milliampere-hours (mAh). For the C21N1725, this is typically around 50Wh or 6500mAh. (C22N1720 Battery for ASUS ZenBook S UX391 UX391UA - 7.7V 50Wh Li-ion Ultrabook Battery, n.d.)
- Chemistry: It is a Lithium-Polymer (Li-Po) battery. (Zenbook S UX391 - Tech Specs, n.d.)
Purchase batteries only from reputable sellers or official distributors. Low-cost third-party batteries may have reduced capacity, shorter lifespan, or present safety risks. (Are Third-Party Laptop Batteries a Safe Alternative to OEM Batteries?, 2023)
How to Replace Your Asus UX391 Battery
Once you have your new C21N1725 battOnce you have the new C21N1725 battery, proceed with replacement. The process is straightforward with careful attention to each step. If you've already opened your laptop to check the battery model, you're halfway there.
- Disconnect the Old Battery: Before touching anything else, you must disconnect the battery from the motherboard. The battery connects via a small cable with a snap-on connector. Gently hold the wires and pull the connector horizontally away from the socket. Do not pull on the wires themselves, as they can detach from the connector head.
- Unscrew the Old Battery: The battery is held in place by a few screws. Locate and remove them, making sure to keep them separate from the case screws.
- Remove the Old Battery: Carefully lift the old battery out of the laptop chassis.
- Install the New Battery: Place your new C21N1725 battery into the same position. Align the screw holes.
- Secure the New Battery: Re-insert the screws to hold the new battery firmly in place. Don't overtighten them.
- Reconnect the Battery: Align the battery connector with the socket on the motherboard and press it firmly into place until it clicks. You should feel it seat securely.
- Replace the Back Cover: Re-attach the bottom cover, ensuring all the plastic clips snap into place. Then, screw the cover back on.
After Installation: Calibrating Your New Battery
To ensure your laptop accurately reports the charge level and to maximize the battery's health, it’s important to calibrate it.
- Charge to 100%: Plug in your laptop and charge the new battery to 100% without interruption.
- Keep it Charging: Leave it connected to the charger for at least two more hours after it reaches 100%.
- Discharge Completely: Unplug the laptop and use it normally until the battery drains completely and the laptop shuts itself off.
- Recharge to 100%: Finally, plug it back in and charge it to 100% without interruption.
Your new battery is now calibrated and ready for regular use.
Tips for Extending Battery Lifespan
A new battery restores your laptop’s performance. With proper care, you can maximize its lifespan and maintain long battery life.
Smart Charging Habits
Modern lithium-polymer batteries do not have a memory effect, but they are sensitive to extreme charge levels. (Sasaki et al., 2013)
- Avoid deep discharges: Try not to let your battery drain to 0% regularly. Plugging it in when it reaches around 20% is a good practice.
- Avoid keeping the battery at 100% charge for extended periods. If you use your laptop plugged in often, consider using the MyASUS application’s Battery Health Charging mode to limit the maximum charge to 60% or 80%. This helps preserve battery health when the laptop is used primarily while stationary.
Optimize Your Laptop's Settings
Adjusting software settings can significantly impact battery life.
- Adjust Screen Brightness: The display is one of the biggest power consumers. Lowering the brightness to a comfortable level can save a surprising amount of energy.
- Use Windows Battery Saver Mode: When you're running low on power, activate the Battery Saver mode in Windows. It automatically limits background activity and push notifications to conserve energy.
- Manage Startup Programs: Too many applications launching at startup can drain power and slow down your system. Disable unnecessary startup programs through the Task Manager.
- Close unused applications and browser tabs to reduce power consumption and maintain system efficiency.
Your Laptop, Re-Energized
Replacing the battery in your Asus ZenBook S UX391 is a cost-effective way to restore performance and portability. By selecting the correct C21N1725 model and following proper installation, you can extend your device’s useful life. Combined with smart charging habits and optimized settings, your new battery will support productivity and mobility for years.
Don't let a dying battery tether you. Do not let a failing battery limit your mobility. Take action to restore your laptop’s performance. Replacement Guide (C21N1725)